1. Field of the Invention
The present invention generally relates to systems for signal coding, and more particularly to a signal coding system utilizing a unique multilevel coding method for achieving improved bit error rates.
2. Discussion of the Related Art
As is known, many modems communicate digital data over an analog medium by mapping the data onto analog signals. Many sophisticated modems communicate digital information by mapping the data onto analog signals and, thereafter, modulating an analog carrier with those signals. Typically, a collection of bits are combined form a two-dimensional symbol, which symbol components are converted to analog form, and the analog-form components respectively modulate a carrier and a quadrature replica of the carrier. The two modulation products are added and filtered, and the result is applied to the transmission medium. A remote modem receives the signal, samples it, identifies the magnitudes and phases of the analog samples, converts the samples to symbols, and finally recovers the original bits of data. What these modems do, in effect, is encode the digital signals onto a two dimensional symbol constellation that is modulated onto a carrier.
Due to noise, imperfect channel conditions, and other factors that are known, there should be a way to help ensure the proper communication of signals across a transmission channel. In this regard, signal coding, or channel coding, has long been know as a way to protect the integrity of data transmitted through a channel. Through the implementation of coding, unsystematic redundancy can be removed from message signals so that channels can be used with maximum efficiency. In addition, through the use of coding, systematic redundancy can be introduced into the transmitted signal so that errors caused by noisy channels can be corrected. Stated another way, the ability to detect and/or correct errors resulting from a noisy transmission channel can only be provided by the additional transmission of redundant bits, and thus by lowering the effective information rate for a given transmission bandwidth.
A fundamental axiom which has motivated the development of coding methods, known as Shannon's coding theorem, provides that if a source generates information at a rate that is less than the capacity of a transmission channel, there exists some encoding procedure that will permit the source output to be transmitted through the channel with an arbitrarily small probability of error. Multilevel coding of signals is one coding method known for achieving increased transmission speeds. However, multilevel signals are more likely to be corrupted by channel noise. Therefore, error-correcting schemes are generally implemented in systems having multilevel signals.
One example of a multilevel coding system is presented in U.S. Pat. No. 5,659,579, invented by the inventor of the present invention. The system disclosed in that patent recognized the fact that some components of a symbol are more susceptible to errors (due to noise) than other components. Accordingly, those more susceptible components (e.g., least significant bits) are encoded more robustly than other components.
As is known, system design is largely performed by making a number of tradeoffs. For example, error detection and correction capability may be added to a system at the expense of added redundancy. Although the amount of information communicated is constant, more actual data is communicated. Likewise, as the encoder design becomes more intense or robust, system complexity is increased. In multilevel systems, additional protection may be obtained through additional coding layers. However, additional coding layers add delay to the system at the receiver/decoder.
Accordingly, there is a need for a system that provides improved performance through coding, while minimizing the system drawbacks or tradeoffs that must be made.